1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, and more particularly to such apparatus capable of detecting image forming conditions and controlling image forming means by the output of said detection.
2. Description of the Prior Art
There are already known copiers in which an image is formed on a recording sheet by means of a photosensitive drum and through steps of charging, exposure, image development and image transfer.
In a conventional copier in which the photosensitive drum and the developing sleeve are driven by a common driving source, the toner is deposited from the sleeve onto the photosensitive drum according to the potential thereof, since said drum and sleeve are simultaneously driven.
In consideration of such drawback, there is already employed, in the conventional copier, a method of applying so-called blank exposure to the non-image area of the photosensitive drum to prevent such toner deposition. Also there is employed a method of switching the developing bias voltage to a value not causing such toner deposition.
However, in a copier equipped with an automatic control mechanism for stabilizing the image density by detecting the potential of the latent image with a potential sensor and controlling the image forming conditions by the result of said detection, it has been necessary to employ a blank exposure lamp of a high intensity with an accordingly high power consumption or to employ a high developing bias voltage for the negative toner in order to prevent the toner deposition in the non-image area, since the non-image area eventually contains not only the light area potential but also the dark area potential, because of the facts that such light potential and dark potential have to be both formed on the photosensitive drum for image density control and that the blank exposure lamp requires a long stabilizing time and is inevitably associated with considerable fluctuation in the light intensity.
As an example, in a copier employing a CdS photosensitive drum with a dark potential of +500 V, a bias voltage of +600 V is given to the developing roller for preventing the toner deposition in the non-image area.
On the other hand, the photosensitive drum is known to show a certain developing characteristic, called reversal development, as shown in FIG. 1 in relation to the surface potential, wherein the toner starts to be attracted to the photosensitive drum (for zero developing bias) if the surface potential of the drum exceeds a certain value (-500 V in case of FIG. 1) even when it is repulsive in polarity to the charge of the toner. Consequently, if the developing bias is fixed for example at +600 V as explained before, a considerable amount of toner is consumed by deposition to the photosensitive drum in the non-image area, corresponding to a low potential area or a lighted area.